1. The Field of the Invention
The present invention relates to Vertical-Cavity Surface-Emitting Lasers (VCSEL). More specifically, exemplary embodiments of the present invention relate to VCSELs with an integrated optical filter.
2. Related Technology
Computing and networking technology have transformed the world. As the amount of information communicated over networks has increased, high speed data transmission has become ever more critical. Many high speed data transmission networks rely on fiber optic networks, due to the large bandwidth of fiber optics and the ability of fiber optics to handle high speed data transmissions. Fiber optic networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to those that form the backbone of the Internet.
Typically, data transmission in such networks is implemented in part through the use of an optical transmitter such as a laser. The optical transmitter emits light in response to a drive current and the intensity of the emitted light is a function of the current magnitude. Data reception in such networks is generally implemented by way of an optical receiver, an example of which is a photodiode. Particularly, the optical receiver receives an optical signal and generates a current, where the magnitude of the generated current is a function of the intensity of the received optical signal.
In many fiber optic networks, it is often desirable to use directly modulated laser (DML) sources as the optical transmitter in order to lower overall system cost. Examples of directly modulated lasers include Vertical-Cavity Surface-Emitting Lasers (VCSELs), Fabry-Perot (FP) lasers, Distributed Feedback (DFB) lasers, and Distributed Bragg Reflection (DBR) lasers.
Among directly modulated lasers, VCSELs are often the lowest cost to implement. The light forming cavity of a VCSEL is usually formed by epitaxially grown distributed Bragg reflectors, and the emission of laser light occurs in a direction that is normal to the laser epitaxial plane, or vertical. This vertical geometry enables high processing yield, low-cost on-wafer laser testing, and tight emission angles.
Commercial VCSELs for fiber optic communication applications commonly have operating wavelengths that are relatively short, near 850 nm, and thus have been limited to relatively short distance data communication through multi-mode fibers. It has proven much harder, however, to use VCSELs in fiber optic communication applications requiring wavelengths of 1310 nm and 1550 nm, which are more desirable wavelengths for single mode fiber long haul communication applications. More recently however, progress has been made in using VCSELs for long haul communication applications.
For traditional single mode fiber communications, directly modulated edge-emitting lasers such as FP, DFB, and DBR lasers have typically been used for applications requiring 1310 nm and 1550 nm wavelengths. Of the directly modulated edge-emitting lasers, FP lasers are typically the lowest cost to implement and are capable of multi-longitudinal mode emission. However, because of the fiber absorption and dispersion in the transmission fiber, FP lasers are generally limited to use for shorter distances, or in lower data rate applications over single mode fibers
On the other hand, DFB and DBR lasers are relatively more complicated and expensive to make than FP lasers, but the single longitudinal mode behavior that results from a grating or a DBR mirror in the DFB and DBR laser active regions enables a light signal to propagate much further in single mode fibers than when using the FP laser.
Many directly modulated lasers, however, lack sufficient performance for higher speed and longer distance links such as, for example, at 10 Gb/s or greater and 40 km and above, due to a large chirp inherent in directly modulated lasers. In particular, the wavelength of the directly modulated lasers changes slightly when the lasers are modulated by signal data. This change in wavelength, or chirp, causes optical frequency distortion in the optical fiber, resulting in corruption of the transmitted data signal.
One approach to the chirp problem in high speed, long-haul applications is the use of electro-absorptive modulator lasers (“EML”) or other light sources with external modulators to reduce the chirp and extend the distance. However, such an approach requires the use of expensive EML chip and packaging, or external modulators and electronic controls, which add to the overall cost of the system.
Another approach to the chirp problem is to use directly modulated lasers in conjunction with an external narrow-band optical filter to convert frequency modulation, or the chirp, to amplitude modulation. However, this approach of using an external optical filter with a DML is still very expensive to implement due to the complex packaging and additional equipment and electronic control of the system.